I really love working with ground rails and cavalettis to work on ride ability and suppleness. Some days, I will just set up three in a row at "normal" distances (depending how big the ring is, they may each be a normal 6 stride). Then I practice asking them to leave a stride out or add a stride. With the young horses, I will start with ground rails and advance them to cavalettis when I think they are ready. For a slightly more advanced exercise, I will put the ground rails/cavalettis in a circle, and work on adding/leaving out strides on the circle. The important thing for all exercises with young horses is to be very clear about what you are asking, but to make sure to have patience and never get upset with them for their greenness. Horses only know as much as WE teach them, so their understanding of the exercise is our responsibility. If they are not understanding it, there is something wrong in our aids and communication. If you are getting frustrated, just simplify the exercise by taking one element out until they can master just two rails. Lastly, recognize that any progress is progress and they should be rewarded for even very small improvements.